The inventive subject matter relates to semiconductor memory devices, and more particularly, to memory devices having variable resistance memory cells.
Semiconductor memory devices are used to store data. These devices may be classified as volatile semiconductor memory devices and non-volatile semiconductor memory devices. Volatile semiconductor memory devices may store data by charging and discharging a capacitor, and include semiconductor memory devices such as a random access memory (RAM). Volatile memory devices typically store and read data when power is applied and may lose data when power is interrupted. Volatile semiconductor memory devices are commonly used, for example, for main memory of a computer.
Nonvolatile semiconductor memory devices typically retain data even when power is interrupted. Nonvolatile semiconductor memory devices are commonly used to store programs and data in a wide range of applications, including computers and portable communication devices.
To address demands for increased memory capacity and low power consumption, next-generation non-volatile memory devices of various types have been researched. For example, a phase change random access memory (PRAM) (which uses phase change materials), a resistance random access memory (RRAM) (which uses materials having variable resistance such as transition-metal oxides), and a magnetic random access memory (MRAM) (which uses ferromagnetism materials) are attracting attention as potential next-generation memory devices. The memory storage materials used in such devices share common characteristics, such as resistance that is variable depending on magnitude and/or direction of applied voltage and/or current, with the resistance being maintained without requiring sustained applied voltage and/or current (e.g., refresh is not required).
Memory cells in such resistive memory devices may include a resistive element and a switching element. The switching element commonly is connected between variable resistance element and a word line. Resistive memory device typically include an array of such resistive memory cells.
Resistive memory cells may be classified at PRAM, RRAM, MRAM, etc., depending on materials used for the resistive element of the memory cell. For example, the resistive element of a PRAM cell may include phase change materials such as Ge-Sb-Te, resistance of which is varied depending on temperature. The resistive element of a RRAM cell may include a top electrode, a bottom electrode and transition-metal oxide (complex metal oxide) between the top and bottom electrodes. The resistive element of a MRAM cell may include a magnetic top electrode, a magnetic bottom electrode and dielectric materials between the top and bottom electrodes.
Resistive memory cells may be classified as a unidirectional resistive memory cell, in which its resistance changes depending on the magnitude of the applied voltage or current, or a bidirectional resistive memory cell, in which its resistance changes depending on the direction as well as the magnitude of the applied voltage or current.